Polycarbonates are inherently electrically insulating. Polycarbonate articles, particularly those with a high surface area such as films or sheets, are therefore generally prone to the accumulation of a surface static electric charge (also referred to as surface static charge), which may not dissipate rapidly. Such accumulated charge attracts dust and dirt from the environment, which can reduce the transparency of a clear plastic article, or reduce the gloss or surface finish of an opaque or colored part, and thereby impart a low aesthetic quality to the part. In addition, in optical film applications, the attracted dust and static cling between films may reduce the product yield during the various cutting and assembly processes.
Antistatic agents can be used to reduce surface static charge. Antistatic agents (also referred to herein as “antistats”) may be ionic or non-ionic in nature, and generally include a hydrophilic head and a hydrophobic tail portion. The hydrophilic head of the antistat forces the molecule to migrate to the surface wherein hydrogen bonding with atmospheric moisture can form a conductive surface layer of water, which promotes the dissipation of electrostatic charges. As a result, dust particles are attracted to a lesser extent, and the fixation of dust is significantly diminished. Ionic antistats generally have poor heat resistance compared to non-ionic antistats, but generally have good antistatic properties.
Antistatic agents can also be applied to the surface of a finished plastic part by spraying, wiping or dipping. Such antistatic agents are referred to as external antistats, and in general they have a pronounced and immediate effect on dissipating charge. However, such antistats are susceptible to accidental removal by abrasion, or by environmental agents, and therefore to maintain antistatic behavior in the part, reapplication is usually needed.
Antistatic agents can also be added to the polycarbonate compositions prior to processing (e.g., extruding, molding, etc.). Incorporated in this way, the antistatic agents are usually referred to as internal antistats. Internal antistats therefore need to be thermally stable to withstand polycarbonate processing conditions. It is also desirable that these antistatic agents be able to migrate to the surface of the article during processing so as to impart the most effective antistatic decay behavior. Additionally, the incorporation of antistatic agents should preferably not diminish desirable characteristics of the polycarbonate material such as transparency. The use of internal antistatic agents is generally preferable to using external antistatic agents as they offer longer-term protection and there are no additional processing steps required after manufacturing a finished plastic part.
Phosphonium salts of certain sulfonic acids have been used as antistatic agents for polycarbonates. For example, U.S. Pat. No. 4,943,380 discloses the reduction of the static charge on polycarbonate resin with an alkylphenyl sulfonate phosphonium salt. A drawback of alkylphenyl sulfonate phosphonium salts is that high concentration levels are required for satisfactory antistatic behavior, which can lead to deterioration of other desirable properties of polycarbonates such as haze and/or transparency, impact strength, or melt flow rate.
U.S. Pat. No. 6,194,497 discloses antistatic resin compositions, including transparent resin compositions comprising perfluoroalkyl sulfonate phosphonium salts as antistatic agents. In the above-mentioned patent, it is shown that tetrabutyl phosphonium perfluorobutyl sulfonate has better antistatic properties at significantly lower concentration than tetrabutylphosphonium dodecyl benzene sulfonate (trade name EPA-202 antistat, available from Takemoto Oil & Fat Co.). EPA-202 antistat is the antistatic agent disclosed in U.S. Pat. No. 4,943,380. For example, it has been found that a polycarbonate composition having 0.8 wt % tetrabutyl phosphonium perfluorobutyl sulfonate has a surface resistivity that is equivalent to that of a polycarbonate composition having 2 wt % EPA-202 antistatic agent. Non-phosphonium containing perfluoroalkyl sulfonate salts such as alkali metal perfluoroalkyl sulfonate salts have not been recognized in the art as antistatic agents but have been found to impart improved flame resistance (FR) to polycarbonates as recognized in U.S. Pat. No. 3,775,367.
Although the use of perfluoroalkyl sulfonate phosphonium salts provides good antistatic properties at relatively low loadings, there still remains a need for improving the effectiveness of antistatic agents in thermoplastic compositions. In particular, there is a need for thermoplastic polycarbonate based compositions having a low content of antistatic agent with superior static dissipative characteristics, in which the optical properties are maintained.